I have a 2005 LLY/Allison with only 17k original miles. The truck has had a Diablo predator tuner on it at least half it's life. Last few yrs it's been ran on the 85HP tune. Last yr I was driving down the interstate and decided to romp on it and soon as I did it locked it's self in I think 3rd gear and threw engine light. I pulled over cleared engine light with tuner and back to normal so I'm assuming I limped it. Well it did it couple more times the next couple times I had it out and wasn't really hard to hard on it but cleared engine light and it was fine. So I took it home wash waxed covered it up and it was not moved until a yr later which was about a week ago. I bought a trans am so I decided I was going to set the truck back to stock to preserve it which I did before I moved it. As I took off in truck it was shifting pretty hard but didn't think much of it at the time as I figured it was where it wasn't use to the lower power. Well I get about 15 miles down the road the engine light came on and I noticed it was locked in the gear. I wasn't the least bit hard on it but I figured I would be able clear light again and go on but I couldn't get it to clear off and now while sitting when I go to put the truck in gear when it goes into reverse or 1st it jerks and clunks violently every time with SEL on and it will not shift out of 1st gear. I messed with it for couple hrs then I finally got it to start shifting smooth into gear and shift threw gears pretty decent still firm for a few miles then the sel came on again and won't shift out of 1st again and violently jerking when moving shift lever into reverse or 1st . Took to gm dealer let it sit overnyt the next day they reflashed the computer and took it for about a 10 mile drive said it shifted ok kinda hard thru gears but smoother when giving it more throttle and soon as he got back to dealer the SEL came on again and was back to not shifting out of 1st and violently jerking clunking when moving shift lever into reverse or 1st. He hooked scanner up to it then and got code p0748 from it. They said they don't have a tranny tech too work on an Allison. So I went today and picked truck up which it went into gear smooth and shifted thru gears pretty decent but few miles down road it locked into the gear I was in and wouldn't shift from 1st once i stopped and was back to violently jerking while moving gear selector into gear. Any idea what is wrong with the truck? With 17k miles I'm hoping it's something fairly simple to fix. Any help is appreciated
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